Method of making a heat exchange tube

ABSTRACT

A method of placing a heat exchange tube into compressive contact with a concentric fin annulus involving a displacement of metal by electromagnetic or equivalent forces to achieve a conforming relation of the tube to the contact tube surface in a manner providing minimal contact resistance to heat flow.

0 United States Patent [151 3,636,607

DeMarco 1 Jan. 25, 1972 [54] METHOD OF MAKING A HEAT 3,267,559 8/1966 Laux ..29/157.3 x EXCHANGE TUBE 3,088,200 5/1963 Birdsall et al.... ..29/42l M I 1,840,724 1/1932 Koehring ..29/l57.3 A UX Inventor Samuel J- DeMarw, Findlay, 0hl0 2,692,763 10/1954 Holm "29/1573 A ux [73] Assignee: United Aircraft Products, Inc., Dayton,

Ohio Primary Exammer-lohn F, Campbell I Assistant ExaminerDonald C. Reiley, Ill [22] Med: Dec. 30, 1969 Attorney-J. E. Beringer [2l] Appl. No.: 889,123 ABSTRACT [52] U S Cl 29/157 3 R 29/157 3 D 29/42 M A method of placing a heat exchange tube into compressive ,29/455 -29/5 l6 29/5 contact with a concentric fin annulus involving a displacement [5 l] Int Cl 821d 55/02 5 [5/26 of metal by electromagnetic or equivalent forces to achieve a [58] Fie'ld 3 A 42] 516 conforming relation of the tube to the contact tube surface in D 2 3 H3/l18 a manner providing minimal contact resistance to heat flow.

[56] References Cited 9 Claims, 7 Drawing Figures PATENTED M25 i972 INVENTOR SAMUEL J. De MARCO ms 1 romve'r FIG-4 2 METHOD OF MAKING A HEAT EXCHANGE TUBE BACKGROUND OF THE INVENTION This invention relates to the art of heat transfer and more particularly to a method of fabrication of a tube assembly comprising a tube an an interior fin.

Tube assemblies of the kind to which the invention pertains comprise a tube usually arranged in a bundle with other like tubes for a flow of a first fluid over and around the tube exteriors. A second fluid, different in temperature from that of the first and separated therefrom, is passed through the tubes. An exchange of heat occurs through the tube walls. A strip of corrugated metal, termed a fin, is contoured to an annular shape and inserted in the tube where it is in common contact with the tube wall and the internally flowing second fluid. The fin is an extended surface member, supplementing the tube wall in achieving a conduct of heat. It has an efiiciency determined in large part by the excellence and continuity of its contact with the tube wall.

In the prior art it has been known to slip fit a fin within a tube. Also, it has been known metallurgically to bond a fin within a tube, as by soldering or brazing. In another known method, an inner tube is placed within the fin annulus and a mandrel drawn through the inner tube to expand it and thereby seat the fin to the surrounding wall of the outer tube. All of the known methods suffer from defects which are reflected in less than optimal heat transfer and a need for an excess amount of heat transfer surface to perform to given specifications. Most importantly they are to various degrees defective in not achieving uniformly minimal contact resistance to heat transfer between the fin and the tube wall. To whatever extent the fin lightly engages the tube wall or does not engage it at all there is a buildup of contact resistance, reducing the facility with which heat may flow from the fin to the tube wall or vice versa. Conversely and in accordance with the known principles of contact resistance, a firm pressural contact of the fin with the tube wall reduces the resistance to heat flow. In a slip fit construction, there is no pressured engagement of the fin with the tube wall. Metallurgical bonding gives good thermal conductivity but may be difficult to achieve in a confined fin tube. Gaps in the created joints, as may result from irregularities in fin height, produce nonuniform heat transfer effects. In a method expanding an inner tube upon an intervening fin, the applied compressive forces have inconsistent results because of the practicable impossibility of forming fin strip material to precisely uniform heights. Also, some types of fins advantageously used in heat exchange tubes have built in wave form in their tube contacting surfaces producing an inherently interrupted contact with the tube wall. A mandrel drawn through an inner tube may have a capability of compressing some of the fin surface upon the outer tube wall but cannot uniformly compress the fin thereon irrespective of variations in fin height or of fin configuration. A tube assembly fabricated by the expanded inner tube method will have numerous areas of individually greater or lesser extent in which the fin contacts the tube wall lightly or not all all. Heat transfer in these areas is at a low value, reducing overall heat transfer efficiency in the tube. Moreover, such areas are vulnerable to deformation by high-pressure fluid in a manner to progressively broaden noncontacting areas.

Expanding of the inner tube is, moreover, an indirect means of compressing the fin material to the outer tube wall. Since contact resistance at the outer tube wall is of chief concern in a tube assembly of the class described, expansion of the inner tube is a less effective means of reaching the desired end than would be contraction of the outer tube. This, however, is a problem of less-obvious solution and the use of a die or the like is in any event subject to the same disadvantages as the forcing of a mandrel through the inner tube. The die would of necessity be fixed in diameter and incapable of compelling the tube to deform in accordance with deformities of the fin surface.

SUMMARY OF THE INVENTION The present invention has in view a method of fixing a fin annulus within a tube involving metal displacement, but which is not limited by the geometry of a particular die or mandrel. A process of unison deforming is proposed in which an outer or an inner tube is displaced radially against the resistance of the fin annulus. The tube wall is allowed to flow freely into a conforming relation with the fin surface with the result that the entire fin strip is placed uniformly under compression irrespective of undulations and irregularities in fin height. Contact resistance is reduced to minimal levels and is uniformly low over the length of the tube. According to a feature of the invention, tube wall displacement is a function of applied electromagnetic forces and may be reflected in either an expansion of an inner tube or contraction of an outer tube. According to a further feature, shrinking of the outer tube is contemplated in order that direct action may be exerted upon confronting surfaces of the outer tube and the fin material, the electromagnetic forming process lending itself readily to an arrangement in which an assembled tube is bodily received within a forming coil.

Objects of the invention are to provide an improved method substantially of the class described resulting in improved utilization of the principle of contact resistance, specifically advancing the pertaining segment of the heat transfer art.

In the drawings:

FIG. 1 is a detail view in perspective of an outer tube comprised in a tube assembly produced in accordance with the method of the invention;

FIG. 2 is a view in perspective of an inner tube useful alternatively as a backup means and as a means to apply a compressive force;

FIG. 3 is a view in perspective of a length of fin material as interposed between the inner and outer tubes;

FIG. 4 is a view in longitudinal section, partly diagrammatic, showing a tube assembly within an electromagnetic forming coil and indicating the direction of applied force;

FIG. 5 is a detail fragmentary view like FIG. 4, showing the parts after completion of the forming operation;

FIG. 6 is a fragmentary view of a tube as shown in FIG. 5, enlarged with respect thereto; and

FIG. 7 is a view in cross section taken through an assembled tube.

Referring to the'drawings:

A tube assembly as achieved by the method of the invention includes an outer tube 10 made of a metal of good thermal and electrical conductivity. The tube is of a uniform diameter, is relatively thin walled and is open at both ends. Also comprised in the tube assembly is an inner tube 11 which may be constructed like the tube 10 but which in any event provides a relatively unyielding exterior for the seating thereon of fin material as will hereinafter more clearly appear. In the illustrated instance the inner tube is designed solely for use as a backup means for the fin material. At least one end thereof is closed, as by crushing or pinching an end 12.

Completing the tube assembly is a strip 13 of fin material. The strip 13 is comprised of a thin gauge, ductile metal of good conductivity. Originally in sheet form, it is gathered and crimped to a corrugated formation to define a series of parallel fins 14 of longitudinal extent. Each fin comprises a peak portion 15 connected by inclining beam portions and vertically spaced valley portions 17 to adjacent fins. According to a feature of the invention in its illustrative form, the fins 14 are formed with a ruffled configuration, that is, with continuous undulations from side to side along the length thereof. A fluid flowing through the fins accordingly is subjected to repeated changes of direction with turbulent effect yielding increased heat transfer efficiency. As a product ofthe laterally formed undulations in the fins, upper and lower surfaces of the fin strip are simultaneously given a wave form in which high points 18 and low points 19 succeed one another in alternating relation from end to end of the strip. In its production, efforts are made to maintain the fins 14 of a uniform height over the full area of the strip. Manufacturing limitations preclude completely consistent results in this connection, however, so that in addition to the alternating high and low points 18 and 19 fin material may have other areas which are greater or lesser in height than adjoining areas.

The method of the invention involves a preassembly of the parts in which fin strip 13 is rolled to an annular configuration about inner tube 11 with these parts then being inserted as a subassembly into outer tube 10. With the parts so positioned, the strip 13 has an outer surface presented for contact with the inner wall of tube 10 and an inner surface presented for contact with the exterior of inner tube II. The parts will have a relatively close fit in which they are frictionally held in an assembled relation. High points on the fin strip will at least lightly engage confronting tube surfaces, but the relationship of the fin surfaces to the wall surfaces is irregular and numerous locations exist of high-contact resistance, as indicated in FIG. 4.

Referring to FIG. 4, a preassembled tube is inserted in an electromagnetic forming coil 21. This is a device storing and releasing electrical energy which assumes the form of magnetic pressure with respect to a workpiece. In this instance, the forming coil has a cylindrical shape and is suitably connected to a power source to draw energy for a period of seconds, store it and then release the energy in a fraction of a second to do work at a high-energy rate. The coil is constructed to have a length exceeding that of the tube so that a fully inserted tube is completely contained within the coil which overlaps the ends thereof. Approximately centered within the coil, the tube is completely and uniformly subject to the magnetic field exerted by the discharging coil 21. An adapter 22, made of nonconductive material, is disposed in the coil 21 to receive and position a tube within the coil. In the process, electrically conductive outer tube 10 becomes the work piece. It is subjected, in response to release of the stored electrical energy, to a force proportional to the intensity of the magnetic field and current. The generated force results in a movement of the conductor, in this instance the tube 10. The coil, since it completely. surrounds the tube 10, applies a force directed radially inwardly so that the material of the tube is displaced in this direction resulting in a reduction in tube diameter. However, since the tube is completely surrounded by the windings of coil 21 all parts of the tube are independently and equally responsive to the inwardly directed pressure. However, while the tube is uniformly affected by the electrical discharge it is free to conform to the underlying surface against which it is pressed. In accordance with the present inventive concept the tube is not compressed upon a fixed die but rather upon a relatively compressible member as defined by the fin strip 13. The contracting tube applies a compressive pressure to the fin material, squeezing it between the inner wall of the outer tube 10 and the outer surface of inner tube 11. Peaks l and valleys 17 are to a limited extent distorted by such compressive forces but the beamlike walls 16 provide effective resistance inhibiting a uniform crushing of the fin. Fluid passageways as defined by the fins l4 accordingly remain open. Also, the tube is constrained thereby the flow into a conforming relation to the high and low points 18 and 19 of the tin surface as well as to low areas existing in consequence of manufacturing tolerance. The tube effects a glovelike fit with the fin strip 13 and may assume externally an irregular or undulating configuration matching that of the fin strip.

The parts are shown in FIGS. 5, 6 and 7 in the position they assume following application of the electromagnetic forces. As indicated in FIG. 5, the tube 10 has decreased in diameter, shrinking slightly away from adapter 22 and assuming an irregular configuration, shown in greater detail in the expanded segmental view of FIG. 6. For purposed of illustration, the undulating configuration of the tube has been somewhat exaggerated. However, the conforming configuration assumed by the outer tube does in fact exist and is effective to apply a substantially uniform compression to the entire fin strip. The strip is accordingly subjected over its entire area to a firm, pressural contact with the outer tube wall assuring a low, uniform level of contact resistance.

The details of means for energizing coil 21 are not shown as being unessential to an understanding of the method of the invention. They may include a suitable charging circuit, switching and capacitor means. Arrows 23 in FIG. 4 illustrate how current discharging from the coil 21 is directed as an inwardly forming pulse upon the tube 10.

A formed tube assembly is withdrawn from the forming coil and is ready for use. Installations thereof may include one in which a plurality of formed tubes are disposed in a bundle for flow of a first fluid over and around the tube exteriors. A second fluid, in separated relation to the first, is controlled and directed to pass longitudinally through the tubes. The flow is in a annular form, inner tube 11 being closed. The flowing fluid is in contact with the inner wall of tube 10 and is also in contact with the material of fin strip 13. Assuming the fluids to be of different temperature, a transfer of heat takes place through the tube wall. The tube wall serves as primary heat transfer surface with respect to the second fluid in contact therewith. The material of fin strip 13 acts as a secondary surface, conducting heat to or from the tube wall. The uniform tight fitting engagement of the tube wall with the fin surface ensures low-contact resistance for uniformly effective heat transfer.

The invention obviates the need for a fonning die. Fin strip 13 assumes the characteristics of a die, and, since it is compressible, yields for the application of uniform compressive forces. If found necessary or desirable, dielike inserts may be placed in the tube ends to define a fixed diameter at these locations to simplify mounting of the tubes in a header plate or the like. The fin strip 13 has been disclosed as having a ruffled configuration and the invention has special merit applied to such a fin since it provides a method heretofore unknown of achieving excellent contact resistance over the rippling fin surface as defined by high and low points 18 and 19. The problem particularly present in the rufi'led fin is to a degree present in all fin constructions, however, due to a practicable inability to maintain precise height dimensions over the full area of a completed fin. The method of the invention accordingly is used with advantage with fins of other configuration, including straight fins. I

The wave form of the fin strip 13 exists, of course. with respect to both upper and lower surfaces thereof, or with respect to outer and inner surfaces as the strip is bent to an annular configuration. Thus in the partly assembled position of the parts, as shown in FIG. 4, locations of high-contact resistance are present along the exterior of inner tube 11 as well as along the interior of outer tube 10. The electromagnetic forming operation seats the fin strip to the inner tube but with an irregular effect which it has not been attempted to illustrate in FIGS. 5 and 6. In the present instance the concern is with the interface between the fin material and the outer tube 10. Should the reverse be true, that is should the concern be with the interface between the fin material and inner tube ll, then an electroforming coil of expansion effect would be inserted in tube 11. Outer tube 11 would then serve as a back up while tube 11 deforms to a glove fit with the inner fin surface.

What is claimed is:

l. A method of achieving minimal contact resistance to heat transfer between a tube wall and a tube contained fin annulus, including the steps of inserting a compressible fin annulus in a tube, said annulus comprising corrugated thin metal deformable material, providing a central support for said annulus, said support rigidly backing said fin annulus, and applying a radial compressive force to said fin annulus so that all parts thereof are in closely contacting compressive relation to the interior wall of said tube irrespective of irregularities in fin height, the application of compressive forces being accomplished by displacing the tube wall radially inwardly so that all parts thereof are in closely contacting compressive relation to the fin annulus irrespective of irregularities in fin height, the fin annulus reacting upon said central support and the tube deforming to conform to fin irregularities.

2. A method according to claim 1, wherein the displacement of the tube material is accomplished by electromagnetic forming, a tube and inserted fin annulus and central support being mounted within an electromagnetic coil disposing in a fully surrounding relation to the tube.

3. A method according to claim 1, wherein the fin annulus is comprised of thin deformable sheet metal corrugated to produce alternate peaks and valleys, the sheet being formed to a circular configuration for insertion in the tube to present an outer surface for contact with the tube wall and an inner surface for contact with the said central support, inward displacement of tube material being resisted by the fin material with the tube wall flowing to conform to and uniformly compressed undulations in the outer fin surface.

4. A method according to claim 3, wherein the tube wall and the central support present spaced parallel surfaces and said fin material having undulating outer and inner surfaces for respective contact therewith, the displacement of tube material flattening the inner fin surface upon said central sup port and deforming the tube wall into conforming relation to the outer fin surface.

5. A method according to claim 4, wherein the tube displacement is accomplished electromagnetically simultaneously over its full length, each part of the tube yielding to applied forces independently of all other parts.

6. A method according to claim 1, wherein displacement of tube material is accomplished in an electromagnetic coil, a tube assembly comprising a tube an inserted fin annulus and a central support being fully contained within the coil with coil elements in continuously surrounding relation to the tube, the

tube being substantially without rigid backup except as provided by said central support through the intermediately disposed deformable fin material.

7. A method of achieving minimal contact resistance to heat transfer between a tube wall and a fin annulus contained in an outer tube, wherein an inner tube is disposed centrally of the fin annulus in support thereof, characterized by the step of deforming one of said tubes to reduce the annular space occupied by the fin annulus and apply a compressive force to said annulus, the deforming step being an electromagnetic forming process carried out simultaneously over the length of the fin annulus and accomplishing a glove fit of the deformed tube wall to the contacting surface of the fin annulus in which the tube conforms to and adopts a configuration matching irregularities in the fin surface while uniformly compressing said surface.

8. A method according to claim 7, wherein the fin annulus is comprised of corrugated sheet metal material providing adjacent longitudinal channels defined by alternating peaks and valleys and connecting beam walls, said walls occupying substantially radial positions between said tubes, said peaks and valleys being compressed and said walls being substantially incompressible by the deforming tube to require the tube substantially to conform to the encountered fin surface while deforming peaks and valleys insure close fitting intimate contact between the tube wall and the fin annulus for minimal contact resistance.

9. A method according to claim 8, wherein the deformed tube wall is the outer tube, said fin annulus disposing as a compressible means between said inner and outer tubes, the inner tube providing backing support for the fin assembly under the inwardly directed compressive force applied by said outer tube. 

1. A method of achieving minimal contact resistance to heat transfer between a tube wall and a tube contained fin annulus, including the steps of inserting a compressible fin annulus in a tube, said annulus comprising corrugated thin metal deformable material, providing a central support for said annulus, said support rigidly backing said fin annulus, and applying a radial compressive force to said fin annulus so that all parts thereof are in closely contacting compressive relation to the interior wall of said tube irrespective of irregularities in fin height, the application of compressive forces being accomplished by displacing the tube wall radially inwardly so that all parts thereof are in closely contacting compressive relation to the fin annulus irrespective of irregularities in fin height, the fin annulus reacting upon said central support and the tube deforming to conform to fin irregularities.
 2. A method according to claim 1, wherein the displacement of the tube material is accomplished by electromagnetic forming, a tube and inserted fin annulus and central support being mounted within an electromagnetic coil disposing in a fully surrounding relation to the tube.
 3. A method according to claim 1, wherein the fin annulus is comprised of thin deformable sheet metal corrugated to produce alternate peaks and valleys, the sheet being formed to a circular configuration for insertion in the tube to present an outer surface for contact with the tube wall and an inner surface for contact with the said central support, inward displacement of tube material being resisted by the fin material with the tube wall flowing to conform to and uniformly compressed undulations in the outer fin surface.
 4. A method according to claim 3, wherein the tube wall and the central support present spaced parallel surfaces and said fin material having undulating outer and inner surfaces for respective contact therewith, the displacement of tube material flattening the inner fin surface upon said central support and deforming the tube wall into conforming relation to the outer fin surface.
 5. A method according to claim 4, wherein the tube displacement is accomplished electromagnetically simultaneously over its full length, each part of the tube yielding to applied forces independently of all other parts.
 6. A method according to claim 1, wherein displacement of tube material is accomplished in an electromagnetic coil, a tube assembly comprising a tube an inserted fin annulus and a central support being fully contained within the coil with coil elements in continuously surrounding relation to the tube, the tube being substantially without rigid backup except as provided by said central support through the intermediately disposed deformablE fin material.
 7. A method of achieving minimal contact resistance to heat transfer between a tube wall and a fin annulus contained in an outer tube, wherein an inner tube is disposed centrally of the fin annulus in support thereof, characterized by the step of deforming one of said tubes to reduce the annular space occupied by the fin annulus and apply a compressive force to said annulus, the deforming step being an electromagnetic forming process carried out simultaneously over the length of the fin annulus and accomplishing a glove fit of the deformed tube wall to the contacting surface of the fin annulus in which the tube conforms to and adopts a configuration matching irregularities in the fin surface while uniformly compressing said surface.
 8. A method according to claim 7, wherein the fin annulus is comprised of corrugated sheet metal material providing adjacent longitudinal channels defined by alternating peaks and valleys and connecting beam walls, said walls occupying substantially radial positions between said tubes, said peaks and valleys being compressed and said walls being substantially incompressible by the deforming tube to require the tube substantially to conform to the encountered fin surface while deforming peaks and valleys insure close fitting intimate contact between the tube wall and the fin annulus for minimal contact resistance.
 9. A method according to claim 8, wherein the deformed tube wall is the outer tube, said fin annulus disposing as a compressible means between said inner and outer tubes, the inner tube providing backing support for the fin assembly under the inwardly directed compressive force applied by said outer tube. 